Discussion – Our telecommunication-wavelength ORCA quantum memory has demonstrated exceptional SNR performance over a GHz bandwidth. This opens the possibility of interfacing with single-photon sources based on parametric-down conversion or four-wave mixing with modest spectral filtering to match our memory bandwidth, as well as to InGaAs quantum dot photon sources [1]}, [2]} the bandwidths of which are well-matched to our device. Total memory efficiencies approaching unity should be achieved with optimal temporal-spectral mode shaping of the control pulse [3]}, [4]}, [5]}. For some applications, the limitation of our memory is the Doppler-induced storage time, as discussed previously. Magneto-optical trapping of the ensemble would eliminate atomic motion, albeit with the additional complexity of ultrahigh-vacuum chambers. An alternate approach to eliminate the dephasing is to perform velocity-selective optical pumping (VSP) to target atoms with a narrow spread of velocities [6]} or even tailor atomic frequency comb structures for rephasing [7]}. These trapped and VSP approaches however sacrifice significant atom numbers and therefore total memory efficiency. An approach that utilises all atoms is to apply additional optical fields that compensate the dephasing by means of a controlled dynamic AC-Stark shift [8]}, or using additional optical fields to map to an auxiliary state in order to facilitate a backward read out that would automatically undo the accumulated phase [9]}. With Doppler-dephasing eliminated, the storage time would be limited to the decay time of the 4D\(_{5/2}\)  state, of around \(90\,\) ns. Overcoming this would require additional fields to map to longer-lived states, e.g. a higher-lying Rydberg state, or to ground state.
